adjl posted... I'm pretty comfortable assuming that the answer is "however makes things worst for the victims," given how any laws around abortion tend to go.
Conceptually, I'd expect it to go how I believe claiming multicellular children works: If you claim a kid as a dependent, and that kid dies, you can only claim them up to the date of their death. Similarly, I'd guess this just moves the date on which you can start the claim back by 7-8 months, then if you miscarry, you can't claim anymore after the miscarriage date. That seems reasonable enough. In practice, because that effectively amounts to the government paying women to be pregnant for a couple months, I expect there will be extra scrutiny added to prevent people from intentionally miscarrying so they can get a few bucks from the government without having to deliver the kid. Given the extent to which people making this sort of law tend to think being pregnant is a trivial experience, I'd also expect they'd just want the money back because they don't think any parenting costs have been incurred (especially if fathers are also able to claim this credit).
You can claim the fetus as a dependency after you give birth, then you get a tax refund for those 9 months of pregnancy.